Atomic Carbon in M82

We report observations of C I(^3P_1 - ^3P_0) emission at 492 GHz toward the starburst galaxy M82. Both the C I/C II intensity ratio and the C/CO column density ratio are a factor of 2-5 higher than observed toward Galactic photodissociation regions (PDRs) or predicted by PDR models. We argue that current PDR models are insufficient to explain the observations, and propose that some of the emission is due to atomic carbon existing within molecular clouds. Employing new chemical models, which use a fast H_3^+ dissociative recombination rate, we find that enhanced cosmic-ray flux supplied by supernova remnants in the M82 starburst lead to an enhanced atomic carbon abundance and elevated temperatures deep within the molecular clouds, resulting in a higher C I emissivity than found in previous PDR models.